1. Field of the Invention
The present invention relates to a method and apparatus for exposing an interior surface of a stratigraphic sample, and in particular to a method and apparatus which leaves undisturbed the physical and chemical properties of the exposed interior stratigraphy of such sample.
2. Description of the Prior Art
The study of geological strata generally involves the on-site collection of geological samples and the removal of these samples to remote laboratories for analysis. The geological material is typically collected in the form of an elongated cylindrical core which can be encased within a plastic sleeve or tube that is positioned within the boring shaft or pipe. (The term "sample" will hereinafter be used to refer collectively to both such a core and the sleeve in which it is enclosed.) Once in the laboratory, the sample is split along its longitudinal axis to expose the interior stratigraphic profile of the core material in a process which is commonly referred to as "slabbing". The longitudinal section thereby split off from the sample comprising the core material, with or without the section of the sleeve or tube is referred to as a "slab".
The accuracy of the data acquired from the laboratory analysis of the slab depends to a large extent upon the degree to which the interior stratigraphic profile is left undisturbed by the slabbing process. Disturbance to the interior stratigraphy poses a particular problem when the core is composed of material which is "friable" (i.e., brittle) or "unconsolidated" (i.e., composed of loose gravel, sand or the like).
Several prior art methods have been employed for slabbing samples. In one such method the sample is first oriented so that its longitudinal axis is horizontal. A small horizontal knife blade is then passed several times through one side of the sample in a direction parallel to the longitudinal axis. On the initial pass of the knife blade only the plastic tube surrounding the core is cut. On each subsequent pass the knife is guided slightly closer to the central axis of the core, the objective being to divide the core gradually so as not to disturb its internal composition. After one side of the sample has been completely divided, the sample is rotated 180 about its longitudinal axis and the slicing procedure is repeated on the opposite side. The sample is then pulled apart into equal halves, thereby exposing the interior stratigraphic profile.
Among the major disadvantages of the above method are the extensive amount of time required to slab each sample and the limitation of the method to cores of soft material, such as mud, sand, clays and the like. This limitation is due to the fact that the small blade used in this process is incapable of splitting the larger rock fragments which may be found in the core. Instead the blade merely pushes these rock fragments through the core, causing undesired disturbance to the material comprising the core and thereby to the stratigraphic profile. In addition, this method provides no mechanism for supporting the freshly cut core material after each pass of the knife blade. If the core material is comprised of loose sand or gravel, the freshly cut interior surface will merely collapse as the knife passes through the core. This method therefore can not be used to slab unconsolidated core material.
There is a variation of the above method in which a very broad knife blade is passed once through the entire sample. The knife blade is sufficiently broad so that as the slab is separated from the sample it is supported on the knife blade itself, thereby enabling the slabbing of unconsolidated cores. However, this second method has the disadvantage of always requiring the use of a liquid lubricant (such as water, kerosene, brine solutions or the like) between the knife blade and the core. A lubricant is necessary not only to prevent the knife from sticking as it passes through the core, but also to prevent drag on the blade from distorting the interior surface of the slab as the cut is being made. The use of a lubricant is undesirable because of its tendency to distort both the physical and chemical integrity of the core interior. Additionally, the knife used in this second method is incapable of splitting larger rock fragments which may be contained in the cores.
A third method of slabbing is used in which the sample is first frozen in dry ice. After freezing, the plastic tube is removed from the core by the application of several sharp hammer blows which shatter the brittle, frozen plastic. The still frozen core is then wrapped with aluminum foil and placed within a thick-walled cardboard tube. An immobilizing compound, such as epoxy resin, is then poured into the tube in order to cast the core and the core is left to cure. After the curing heat subsides, the core is refrozen.
The frozen and immobilized core is then oriented vertically and divided by drawing a radial blade saw down through the length of the core. The resulting slab is recovered as it falls to the side.
This third method may be used for slabbing core material which is unconsolidated, since the loose contents of the core are held intact by freezing and immobilization. Use of a radial blade saw also enables the slabbing of samples which contain large rock fragments. However, this third method is both time consuming and expensive, requiring the use of both an immobilizing compound and a liquid coolant (such as liquid nitrogen) to dissipate the heat generated and overcome the friction encountered by the radial blade saw. Disturbance to the core interior may occur at several points during the process, including the freezing and immobilizing of the core and the recovery of the unsupported vertical slab as it falls away from the remaining core.
An objective of the present invention is therefore to provide methods and apparatus for slabbing any sample, including one in which the core material is friable, unconsolidated, or contains large rock fragments, without distorting the physical structure or contaminating the chemical composition of the core.
A further objective of the present invention is to provide methods and apparatus by which a sample may be slabbed directly as it is received from the field with minimum disturbance to the interior of the core.
A further objective of the present invention is to provide a method and apparatus for slabbing which does not require the use of liquid coolants or liquid lubricants.
A further objective of the present invention is to provide a method and apparatus for slabbing which does not require that the sample be frozen or immobilized prior to being slabbed.
A further objective of the present invention is to provide a slabbing apparatus which is simple and inexpensive to construct and is reliable in operation.